Method for producing an aluminium component having a coloured surface

ABSTRACT

A method for producing an aluminium component having a coloured surface, and comprises the steps of anodizing the surface of the component and applying the colouring component by electrolysis. The method is characterized in that the electrolyte comprises an Sn salt and a further salt of a divalent metal. Through the combination of two or more metal salts in the electrolytic colouring of anodized layers, a dense and intensely coloured coloration can be obtained on the aluminium surface.

FIELD

The invention relates to a method for producing an aluminium component having a coloured surface, where, in the method, the surface of the component is anodized and a colouring component is applied by electrolysis. The invention further relates to a component of aluminium having a coloured surface that has been produced by the method.

BACKGROUND

Components made from aluminium and from aluminium alloys find broad application in the automotive sphere, as for example in motor-vehicle bodies and also in the context of decorative components, such as trim strips, for example. The surface of the aluminium is generally first provided with an oxidic protective layer by anodic oxidation (anodizing). The oxide layers formed may subsequently be coloured, using organic or else inorganic dyes, for example. A disadvantage of the organic dyes is that they are not UV-stable, and so after a certain time the surface is visually unattractive.

Another method for imbuing the oxide layers with colour is that of electrolytic colouring (Colinal process). The electrolyte comprises a colouring metal salt, generally tin(II) sulfate. With this process, the metal ions penetrate into the pores of the layer, and the metal-filled pores give rise to lightfast colouring through effects of absorption and scattering. Many shades of colour can be obtained by electrolytic colouring; a disadvantage is that intense and uniform coloration is achievable only with relatively high layers of more than 12 μm. Because of the difference in coefficient of expansion between the metal of the component and of the layers lying above it, the applied layers undergo delamination, particularly at high temperatures, and the corrosive resistance of the metal is impaired.

In place of electrolytic colouring, the aluminium components are frequently also powder-coated. A disadvantage is that the powder-coated paint delaminates rapidly and particularly so at edges and angles. Furthermore, the powder coating does not display the typical metal surface which many customers desire.

SUMMARY

It is an object of the invention, therefore, to provide a method for producing an aluminium component having a coloured surface, with the capacity to generate a stable and UV-resistant coloured layer having a great depth of colour. A further object is to provide a component having a coloured layer of this kind, this component more particularly being part of a motor vehicle and very preferably part of the body of a motor vehicle or an ancillary component for mounting on the body of a motor vehicle.

This object is achieved according to the invention by a method for producing an aluminium component having a coloured surface, comprising in particular the steps of:

-   -   anodizing the surface of the component,     -   applying the colouring component by electrolysis, and         where the electrolyte comprises an Sn salt and a further salt of         a divalent metal.

In respect of the component, the object is achieved by a component produced according to the above method.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a coating which can be produced by the method of the invention.

DETAILED DESCRIPTION

In accordance with the invention it has surprisingly been found that through the combination of two or more metal salts in the electrolytic colouring of anodized layers it is possible to obtain a dense and intensely coloured coloration on the aluminium surface. The layers in this case are relatively thin, and so the difference in coefficient of expansion of the coating and of the metal is barely perceptible at high temperatures or else at low temperatures, and the coating remains substantially intact.

The new method is suitable especially for colouring aluminium components of all kinds, with the term “aluminium component” embracing components or other parts made of aluminium and of aluminium alloys. Components may in particular be a component of a motor vehicle and more preferably part of the body of a motor vehicle or an ancillary component for mounting on the body of a motor vehicle.

In a first step of the method, the aluminium component is anodized. Anodizing may take place in a conventional way. As a result of the anodizing, an oxidic protective layer is formed on the aluminium surface. This layer is finely crystalline and has pores, which in the next step accommodate the colouring substance. In the next step of the method, this colouring layer is applied by electrolysis. The electrolyte comprises, in accordance with the invention, an Sn salt as one component, in combination with a salt of a further divalent metal.

The aforesaid divalent metal salt is preferably selected from salts of Ni, Co, Cu, Mn and/or Fe. All water-soluble salts are suitable, especially halides and sulfates. It has been found that with a combination of an Sn salt and a further metal salt, a dense, coloured layer can be generated even when the thickness of that layer is relatively low.

A particular advantage is that a black colour layer can also be obtained. One particularly preferred combination is the use of an Sn(II) salt with an Ni(II) salt, more particularly the use of SnSO4 and NiSO4.

The two salts are added to the electrolyte in a conventional way. The Sn salt is preferably in a concentration of 0.1 to 15 g/l in the electrolyte or the electrolyte solution. The concentration of the further metal salt is preferably between 2 to 200 g/l. Particularly good results in terms of depth of colour and stability are achieved if the Sn salt is in an amount of between 1 and 10 g/l and the further salt in an amount of between 30 and 150 g/l in the electrolyte. The Sn salt and the further metal salt are preferably in a weight ratio of between 1:20 and 1:200, more particularly 1:30 to 1:100.

The colouring electrolysis can be carried out in a conventional way, preferably with application of alternating voltage. Further components of the electrolyte include sulfuric acid, iron(II) sulfate, sulfophthalic acid and/or sulfosalicylic acid. The pH of the electrolyte is preferably between above 1, more particularly <1 to 2. The electrolysis is carried out preferably at temperatures from room temperature up to a slightly elevated temperature of up to around 22° C. The electrolytic colouring takes place customarily with alternating voltage.

A particular advantage of the present invention is that through the combination of two or more metal salts it is possible to obtain a coloured surface having a deep coloration. The thickness of the electrolytically applied layer is preferably between 6 and 11 μm, more particularly between 6 and 10 μm. A further advantage of the low layer thickness is that the electrolysis can be carried out in a very much shorter time than according to the methods known from the prior art; accordingly, it has been possible to reduce the duration of the electrolytic colouring process by 30%.

Implementation of the Electrolysis

An aluminium component anodized by methods known from the prior art was placed in an electrolysis bath. The electrolyte contained 5 g/l sulfuric acid, 100 g/l nickel sulfate*6H2O, 1.7 g/l iron(II) sulfate, 2 g/l tin(II) sulfate and 15 g/l sulfophthalic acid SSA and the temperature was 20° C. The electrolysis was implemented by application of an alternating voltage of 15-18 V. The electrolysis was carried out over a period of 10 minutes. The component was subsequently removed from the electrolysis bath, rinsed and introduced in a known way into a 15-minute hot-deionized-water densification. Here, the anodized layer was densified in a known way by the intercalation of water and the associated increase in volume.

In a final method step, the component from the electrolysis is subjected to the sol-gel process, in which a coating having a layer thickness of 2.5 μm was applied.

Further features and advantages of the invention are evident from the description below of a preferred exemplary embodiment, with reference to the drawing.

In the drawing, FIG. 1 shows a coating which can be produced by the method of the invention.

The colouring component is deposited by electrolysis on an anodized aluminium surface, the electrolyte being a salt solution composed of an Sn salt and a further divalent metal salt. An anodized layer is obtained which comprises aluminium oxide and also the two metals deposited during the electrolysis. Subsequently a top coat is applied by means of sol/gel operation. 

What is claimed is:
 1. A method for producing an aluminium component having a coloured surface, comprising the steps of: anodizing the surface of the component, applying the colouring component by electrolysis of an electrolyte, characterized in that the electrolyte comprises an Sn salt and a further salt of a divalent metal.
 2. The method of claim 1, characterized in that the further salt is selected from salts of Ni, Co, Cu, Mn and/or Fe.
 3. The method of claim 1, characterized in that the Sn salt and the further salt are used in the form of the sulfates.
 4. The method of claim 1, characterized in that the Sn salt is SnSO4 and the salt of a divalent metal is NiSO4, wherein a black colour surface is obtained.
 5. The method of claim 1, characterized in that the Sn salt is in a concentration of 0.1 to 15 g/l in the electrolyte.
 6. The method of claim 1, characterized in that the further salt is in a concentration of 2 to 200 g/l.
 7. The method of claim 1, characterized in that the Sn salt and the further salt are in a ratio of 1:20 to 1:200 in the electrolyte.
 8. The method of claim 1, characterized in that the electrolyte has a pH of <1 to
 2. 9. The method of claim 1, characterized in that the electrolysis is carried out at a temperature of room temperature to 22° C.
 10. The method of claim 1, characterized in that electrolysis is carried out with alternating voltage.
 11. The method of claim 1, characterized in that the component in a downstream step of the method is provided with a sol/gel coating.
 12. The method of claim 1, characterized in that a black surface is produced.
 13. The method of claim 1, characterized in that the layer applied electrolytically has a thickness of 6 to 11 μm, preferably of 6 to 10 μm.
 14. The method of claim 1, further comprising the step of hot deionized-water densification.
 15. The method of claim 1, characterized in that the electrolyte further comprises one or more of the following: sulfuric acid, iron(II) sulfate, sulfophthalic acid, and sulfosalicylic acid.
 16. A component of aluminium which has a black surface and has been produced according to the steps of: anodizing the surface of the component, applying the colouring component by electrolysis, characterized in that the electrolyte comprises an Sn salt and a further salt of a divalent metal.
 17. The component of claim 16, characterized in that the further salt of a divalent metal is a Ni salt.
 18. The component of claim 16, further comprising a sol gel coating layer.
 19. The component of claim 16, wherein component is part of the body of a motor vehicle.
 20. The component of claim 16, wherein the component is an ancillary component for mounting on the body of a motor vehicle. 